If you would like to, you can buy cheap temporary fencing to make an enclosure the shape of an oval, assuming your horse will respect this. They have short white sticks that you push into the ground with your foot, and you can buy any type of roping to string through the slots that are on the electric fence sticks. Here is a link for what I am talkin about. Step-In Poly Fence Post, 48 in. - 3600956 | Tractor Supply Company
You could set up jumps if that is your thing, but without having some kind of enclosure you are limited to more aimlessly riding than working on bending, turning, etc. It doesn't seem like it would, but having an enclosed area to work makes riding more focused so you can work on things if that is what you choose.
You could ride in the open and work on collection, but again your consistency may have to be sacrificed for not having any boundaries where you anticipate turns and have a steady, strait rail you can work off of.
If you go the electrical pole route, tie a string from each corner to the other to make sure the poles in between are straight. I hope this helps!